Re: how about a well-known DOWNGRADE BGP Community? (Was: RTBH Support Across the Industry)

[Saku Ytti via NANOG <nanog () lists nanog org>]

We ran this very config in production while we still had asr9k.

It is slightly confusing as 0, 1 and 2 on CLI are not same in Tomahawk, in HW they are well ordered. But in CLI 
unconfigured 0 is worst, 1 is best and 2 is middle, so can be confusing if you debug in HW level to see where packets 
are going.

  ++ytti
________________________________
Lähettäjä: Lukas Tribus <lukas () ltri eu>
Lähetetty: Wednesday, 05 August 2026 22:32:26
Vastaanottaja: Saku Ytti <saku () ytti fi>
Kopio: North American Network Operators Group <nanog () lists nanog org>; Job Snijders <job () bsd nl>
Aihe: Re: how about a well-known DOWNGRADE BGP Community? (Was: RTBH Support Across the Industry)

> You set 'priority level 1' for NC+AF, 'priority level 2' for BE, and
> leave LE unset on the ingress policy.
>
> This sets the fabric priority, and VoQ will honor it.

I am very glad this is possible on the 9k. Are you certain there are
no negative consequences moving the entire BE traffic into a fabric
priority queue (back-pressure, etc)?


> Mind you, in your scenario, even without ASR9k priorities, the network
> is protected.
> [...]
> Even if ASR9k congests the victim (which it shouldn't with the right
> priority level), the rest of the core is protected, so the blast zone
> is still just the victim.

The *transit* network is technically protected. But the incentive for
the *customer* network to actually announce the scavenger route is
gone.

If the transit network is injecting a scavenger route itself without
involving the customer, sure, then this makes sense for the benefit of
the transit network.

But more likely the transit customer already switched to RTBH instead anyway.


I'm happy to hear this is possible on the 9k.

Lukas
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